Various flat panel displays have heretofore been proposed for producing a television picture or for alphanumeric and computer graphic display purposes. The systems as proposed would have manifest advantages with respect to the size of the image which might be displayed, when compared with conventional television picture tubes in which any increase in picture area greater than an area having about a 25 inch diagonal measurement results in an inordinate increase in bulk and weight. However, the arrangements as heretofore proposed have either been inoperable or have been so impractical or expensive that they have not been used successfully other than for experimental purposes and in special applications.
In certain of the prior art systems, individual cells or units are provided for producing each image spot, each cell or unit being operable independently of the other, with a matrix of row and column conductors being driven electrically to effect sequential operation of the cells in a predetermined raster pattern. Such arrangements have been very expensive to construct and have required circuitry which is complex and expensive.
With regard to particular prior art disclosures, the Watanabe U.S. Pat. No. 3,622,829 proposed a flat panel display arrangement using a gas plasma as a cathode with electrons being extracted from the plasma by a positive-potential mesh to be directed toward a control-grid array. The grid as disclosed is in the form of a set of holes in a substrate with electrodes in parallel strips along one surface of the substrate and with a second set of parallel strips along the other surface of the substrate, in orthogonal relation to the first set. By applying positive potentials to any selected pair of the two sets of electrodes, the electrons are extracted from the plasma and accelerated to strike a phosphor and produce a luminescent spot.
The Watanabe arrangement would have the potential advantage of increased efficiency and brightness, as compared to systems using other types of cathodes, but there would be practical difficulties in attempting to use the system, especially in a large size display. One problem which is not mentioned or recognized in the Watanabe patent is the problem of support of the front and rear walls of the panel, when the panel is of large size. Since the absolute pressure within the device must be quite low, the atmospheric pressure applied to the front and rear walls can produce extremely large forces when the device is of a large size. For example, in a panel which is 30 inches square, the total forces applied to the front and rear walls may be well over 10,000 pounds and such walls would have to be quite thick and heavy.
Another problem with the Watanabe type of design is with respect to the electrical circuitry required to drive all of the electrodes of both sets.
Certain problems with the Watanabe type of design may be overcome in devices using plasma sac scanning. The production of a plasma sac is described in a journal article entitled "A Picture-Display Panel Using a Constricted Glow Discharge", by H. Hori et al, IEEE transactions on Electron Devices, Vol. ED-21, No. 6, June 1974. As described, a plasma sac is caused to be produced on the cathode side of an apertured insulator and by controlling the potential applied to electrodes, the plasma sac may be caused to move from one aperture to another.
The Miyashiro et al. U.S. Pat. No. 3,749,969 also discloses a plasma sac and discloses a two-dimensional scanning arrangement for effecting movement of the plasma sac in a flat panel type of display. To scan a row, a sac is initiated at the start of the row and is caused to move progressively from cell to cell by changes in the potential on a control electrode associated with each of the cells. Such an arrangement has the potential of reducing the complexity, size and cost of the circuitry required to effect a scanning operation. However, the aforementioned problems with respect to the permissible size of the displays are not recognized and dealt with.
In my U.S. Pat. No. 4,130,777 I disclose a scanning means and method for a plasma-sac-type gas-discharge image display panel in which a plurality of electron-beam-generating plasma sacs are simultaneously formed from a gas-discharge plasma or plasmas and in which scanning means are provided for activating in sequence and group-by-group consecutive groups of plasma sacs in a row until an entire row is scanned.
The scanning means and method as disclosed in my aforesaid patent are highly advantageous. However, certain problems with respect to constructing a large size image display panel were not recognized. In particular, my patent discloses the use of a plurality of elongated hollow cathodes on side-by-side relation with there being a limited number of row electrodes associated with each hollow cathode. The front wall of the panel is disclosed as being supported from the forward edges of supporting walls which extend between the electrodes of the hollow cathodes to separate one hollow cathode from the cathodes adjacent thereto, the electrode structure and associated insulators being interposed between such forward edges and the front wall.
With such supporting walls, it would be possible to support the front and rear walls of a panel having a very large size since the spacing distance between one supporting wall to another may be quite small and a large number of supporting walls can be provided extending throughout the entire area of the panel. Thus, the arrangement would not only provide adequate support for the front and rear walls, but would permit such walls to be relatively thin and light in weight.
A problem with such a construction, not recognized in my patent and not recognizable from consideration of the prior art, is with regard to effecting scanning of the rows which are adjacent the supporting walls. It is found that blank spaces are produced in the image for the reason that scanning of rows near such supporting walls is unreliable. After investigation of the problem, it was found that the walls apparently produce a non-uniform field distribution such that in many cases, the plasma sac has a tendency to either move away from the wall or to become extinguished when scanning potentials are applied to the column electrodes designed to produce scanning movement parallel to the wall. It was found that in some cases, the plasma sac improperly moved along a row spaced a distance from the wall rather than a row adjacent thereto.